A nonwoven fabric is a textile structure consisting of a mat of fibers held together with a bonding material. The fibers can be partially oriented or they can be completely randomly distributed. Latex is often used as the binder for the fibers in nonwoven fabrics.
Nonwoven fabrics are popular owing to the simplicity and economy of their production since the traditional weaving operations are not used; hence, less equipment, less space, and fewer personnel are required. Nonwoven fabrics can also be produced from what would normally be considered as waste fibers, and useful characteristics are obtained which may not be provided by woven or knitted fabrics.
Enormous quantity of fibers are consumed annually in applications of nonwoven fabrics such as clothing interliners, filters, automotive door panels, heat and electrical insulation, packaging, sanitary napkins, fillers for quilted structures, wiping cloths, towels, masks, wall coverings, shoe uppers and liners, curtains and draperies, tea bags, simulated leather, gaskets, luggage, ribbons, and diapers.
Conventional carding equipment used in the weaving industry can produce fiber webs of uniform thickness suitable for impregnation with a binder, but it has one drawback: while lengthwise strength is usually good, cross-direction strength is generally poor owing to the staple fibers being laid lengthwise of the fabric or in the machine direction of the material.
To obtain a nonwoven fabric with substantially uniform strength in all directions, random distribution of the fibers has been achieved by several methods. One of the most popular of such methods involves air-laying of the fibers by stripping same from a carded web by means of an air stream which then directs the fibers through a restricting throat which is controlled to adjust the thickness of the resulting web.
A number of methods have been developed for treating randomly-dispersed webs with a binder. Typically, a water-based emulsion binder system is used in which a thermoplastic or thermoset synthetic polymer latex is prepared and a loose web of fibers to be treated is immersed therein using special equipment in view of the structural weakness of the web. The treated web is then dried and cured to effect proper bonding. Alternatively, an aqueous or solvent solution binder system of a thermoplastic or thermoset resin may be used to impregnate the fibrous web.
Still other methods include the application of thermoplastic or thermoset resin powders to the fibers, before or after making a web of same, and passing the web through hot rolls or a hot press to bind the fibers together. Alternatively, thermoplastic fibers having a softening point below that of the base fibers may be interspersed in a web of the latter and sufficient heat and pressure applied, such as by the use of heated rolls, to soften the thermoplastic fibers and bind the fiber network together.
Commonly used latexes for nonwoven fabrics are those prepared from polymers of butadiene-stryene, butadiene-acrylonitrile, vinyl acetate, and acrylic monomers. While the emulsion binder system using latexes is the most popular method of forming nonwoven abrics, the homopolymers, copolymers and terpolymers heretofore used have suffered from shortcomings. Since, for example, the end uses to which the nonwoven fabrics are applied play a major role in determining what polymeric binder is used, it can readily be appreciated that the properties of the polymeric binder are critical.
While the acrylic polymer latexes are presently enjoying significant success due to the fact that nonwoven fabrics bonded therewith are generally soft and have a good hand, they have certain drawbacks, chief among which are cost and wet strength retention. Presently, acrylic latexes are considerably more expensive than the other common latexes and they do not have the desired wet tensile strength which is of paramount importance in applications such as diapers, wiping cloths, mops, shoe innersoles, etc.